Tom Barrett's Blog: WordPress is more than a CMS, it is a PHP Framework
Note: This article was originally published at PHPDeveloper on 12 May 2012.
A software library is a collection of resources used to develop software. [...] A Software Framework is a software library with certain goals in mind [...]. Hopefully theme and plugin developers will be familiar enough with WordPress to see where I am going with this.
He mentions some of the similarities WordPress has with other PHP frameworks including the integration of hooks/filters/plugins, working with core files that are maintained by an external entity and that it's a full-functional CMS out of the box.
From this, I conclude that WordPress is a framework. It provides us with a library of resources that allow us to extend and specialize it into the site and CMS that we want.